Car accidents have the ability to cause much devastation to the body. There are many causes of this, such as broken glass, objects colliding with people inside the car, but the main cause is the acceleration the body undergoes when the car collides with another object of comparable size.
Acceleration The change in velocity divided by the elapsed time a = v / t
Velocity changes a very large amount in a short period of time. This puts the body under enormous stress and causes injury.
- Velocity The displacement divided by the elapsed time v = d / t
Through experiments, its been found that we can handle an acceleration of 20 g (20 x 9.8m/s2 ) without injury. Published safety standards say that we can tolerate accelerations up to 80 g. But the key is to remember that this must be for a very short period of time.
There is something called a Severity Index, which takes into account both the magnitude of acceleration and the length of time during which it is experienced
S.I. = ((alpha)^5/2) t , where alpha is the acceleration in g, and t is the time of exposure in seconds.
What the Severity Index is intended to do is to give an idea of the chance a passenger has of survival in a collision. A proposed specification is S.I. < = 1000 for the human head.
Some rough estimations of SI in everyday life :
- SI for a person falling back on a chair ~ 1
- throwing a baseball fairly hard (40 km/hr) against a wall ~10 000
- hammer hitting a nail in wood ~ 3 000
- hammer hitting a concrete wall ~ 3 600 000
- driving into a concrete wall at 90 km/hr w/ no seat belt ~ 24 000